The invention is directed generally to electroplating of selected portions of articles. More particularly, the invention is directed to a selective electroplating apparatus and method, and an article plated thereby. The invention involves control of electrolyte wetting of the article and production of a generally uniform electrical field adjacent the selected portions. In an important embodiment, selected portions of an inner surface of an elongated bore in a conductive article are electroplated.
The desirability of selectively and uniformly electroplating portions of articles arises from a number of considerations, including article design requirements. Where scarce precious metals, such as gold, are being electroplated, compelling economic and ecological reasons for avoiding unnecessary plating come into view. The present invention is therefor concerned with meeting design requirements for selective electroplating as well as avoiding waste of plating metal due to plating of unnecessary surfaces and non-uniform plating in excessive thicknesses.
Although the invention is useful in plating metals other than gold onto articles other than screw machine contacts, the present discussion will be generally confined to selective gold plating of electrical contacts, particularly screw machine contacts having elongated bores requiring gold electroplating. Thus, when the term "gold" is used hereinafter, it should be understood that, for example, any of the precious metals including gold, rhodium, palladium, irridium, platinum and silver and alloys thereof may be used. Similarly, electrical contacts other than screw machine contacts and electrical devices other than electrical contacts may be electroplated according to the present invention, as may articles useful in diverse unrelated fields such as the jewelry field.
Important advantages of the present invention over prior non-selective electroplating systems can be appreciated by an examination of conventional plating of screw machine type contacts. These contacts, which are manufactured on a screw machine from tubular stock, are generally tubular in overall shape and have elongated bores for engagement with corresponding contacts. Actual electrical continuity upon engagement is established at the entranceway of the bore of the screw machine contact.
It is desirable to plate gold on the entranceway of the screw machine contact bore because of its excellent conductive and wear properties and its resistance to corrosion. Placement of gold at the bore entranceway, the primary point of actual electrical contact, will improve electrical continuity and overall contact reliability.
The method conventionally employed to plate gold on screw machine contacts has been barrel plating. Barrel plating has been relied upon because it partially overcomes the difficulty of plating the irregular surfaces presented at the entrance to the contact bore by coating the entire contact with plating metal.
Barrel plating is well-suited to indiscriminately plating inexpensive metals onto screws and bolts where uniformity of coating thickness is not important. It is, however, ill-suited to plating expensive metals onto irregular surfaces where selectivity and uniformity of coating thickness are highly desired. It is also ill-suited to plating the inner surface of bores in articles subjected to plating.
Barrel plating of gold onto screw machine contacts thus results in considerable gold waste due to indiscriminate plating. It also produces a build-up of plating metal on projecting corners outside of the contact bore entranceway which wastes metal due to excessive deposition in areas remote from the primary electrical contact surfaces. This build-up at projecting corner, which is seen in other electroplating systems, arises due to the increased density of current flow which occurs at projecting edges of the article being plated.
While prior non-selective electroplating systems like barrel plating are undesirable for plating screw machine contacts for many reasons, including plating metal wastage, prior selective electroplating systems designed to reduce wastage also suffer a number of drawbacks. For one, prior selective systems offer no remedy to the build-up of plating metal at projecting corners adjacent the primary plating areas.
One commonly used selective plating technique is a partial immersion system commonly referred to as "dip plating" in which only a selected portion of the article to be plated is actually immersed in the plating solution. This technique is cumbersome and imprecise due to the difficulty of maintaining immersion depth in a rapidly agitating electrolytic bath. This method also suffers from poor selectivity due to plating resulting from splashing of electrolyte onto non-selected article areas.
Another selective plating technique entails the application of a non-conductive plating mask in the form of a paint, tape or other resistive coating to non-plating areas of each article being electroplated. The time consumed and difficulty of obtaining a tight seal between the mask and the article are considerable drawbacks of this technique, especially in the context of electroplating small, non-planar objects like screw machine contacts.
Yet another category of selective plating technique entails directing a stream of electrolyte against the area to be plated in the presence of the requisite charge density to produce plating. This technique, as in the case of partial immersion, suffers the drawback of electrolyte wetting of areas outside of the selected portion and the undesired plating resulting therefrom.